Exactly which song "At Doom's Gate" is based on has been disputed. Usually suggested are Metallica's "Master of Puppets" and "No Remorse". It is also possible that the song is based on a sequence in "The Family Ghost" by King Diamond (the part can be heard approximately 2:05 into the song). Slayer's "Behind The Crooked Cross" at 1:08 and Blackmayne's "Twilight Of Lear" from their 1985 LP Blackmayne is also possible. The riff, almost verbatim, is also featured in D.R.I.'s "Hooked" from their 1987 LP Crossover and Aria's "Это Pок" (English: "This Is Doom" or "This Is Fate") from their 1985 debut album Мания Величия (English: Megalomania), and possibly Bathory'sSadist. The riff is also featured in the "Deviated Instinct" song "Cancer Spreading" heard in 0:15 into the song. This guitar riff is very common, which is possibly the cause of the confusion.
The Doom IWAD file includes two title screen MIDI lumps, d_intro and d_introa. The latter is used when using OPL MIDI playback (ie. on an Adlib or Soundblaster card).